1. Field of the Invention
The present invention relates to the field of nanostructures, to oscillators, and to the application of mass transport and hydrodynamic effects to create oscillatory movement on a nanoscale.
2. Related Art
The relative importance of different forces is scale dependent (Ref. 1). For instance, human beings cannot carry twenty times their own weight, nor walk on water. Yet insects having such abilities are well known (Refs. 2, 3). The explanation lies in the relative scaling of mass, muscle strength, and surface tension with the length scale s. Thus, relative to weight (˜s3), muscle strength (˜s2) and surface tension (˜s) are one thousand and one million times more effective respectively for insects than for humans because of the thousand-fold difference in linear length scale. As the relevant size scale decreases, the relatively feeble scaling of surface tension makes it progressively more important. In the micrometer and nanometer regimes this force can dominate.
Though often an impediment (Refs. 4-7), surface tension has been utilized in some synthetic microsystems. For instance, electric field-induced changes in surface tension (electrowetting) are used to transport and manipulate droplets in microfluidic systems (Refs. 8, 9).
Reference (8) discloses that a mercury drop in a glass capillary tube can be moved by an electrochemical effect called electrocapillarity. A potential is applied across the interface between the liquid metal and the capillary. The liquid metal is contained in an electrolyte, across which a voltage gradient is applied, causing movement of the metal through the tube.
Reference (9) discusses the application of chemical electrowetting using a surface covered with dielectrics. It is suggested that a digital microfluidic circuit could be created in which droplets are created from a reservoir, transported, cut and merged by electrowetting. With the appropriate design of channels, electrodes and control circuits, a total microfluidic analysis system could be built.
Microscale, surface tension-based bubble valves and pumps have been developed for commercially available inkjet printers and lab-on-a-chip implementations (Refs. 10, 11). A MEMS technology (Ref. 12) for three-dimensional assembly uses the surface tension forces created by melting precisely-sized metal pads to rotate released structures off of the substrate plane.
Reference (12) discusses surface tension as a means for carrying out the assembly of microstructures. In one arrangement, the surface tension holding a droplet to a solid, hinged surface is used to rotate the hinge portion. A solder pad is placed across the hinge, and, when melted, causes the hinge to lift up to adhere to the liquid droplet. A procedure to form electrical networks by self assembly is also disclosed (FIG. 20 of Ref. 12).
The degree to which surface tension is exploited by these approaches is limited, however. In the case of electrowetting, the surface tension effect is differential and thus requires high voltage to access relatively moderate forces. In the other cases, the surface tension action is inherently unidirectional.
Pollack et al., “Electrowetting-based actuation of droplets for integrated microfluidics, Lab Chip 2:96-101 (2002) describes the micromanipulation of discrete droplets of aqueous electrolyte by electrowetting. A series of electrodes are used to control drop merging and splitting.
Tas et al., “Scaling Behavior of Pressure-Driven Microhydraulic Systems,” Nanotech 2002 Vol. 1, Technical Proceedings of the 2002 International Conference on Modeling and Simulation of Microsystems, Chapter 3 (2002) discloses a hydraulic relaxation oscillator that was fabricated on silicon glass. Various hydraulic devices were created to recreate, for fluid flow, various properties of electrical flow. For example, a flow restriction was characterized as a “resistor,” and other devices were created to act as fluid “capacitors.” Assembling these in a fluid circuit resulted in the creation of a relaxation oscillator.
Darhuber et al. “Microfluidic Actuation by Modulation of Surface Stresses, App. Phys. Lett. 82(4):657-659 (Jan. 2003) discloses a microfluidic device that can be used to manipulate nanoliter liquid samples. The device uses hydrophilic lanes and specific heating elements.
Velev et al. “On Chip Manipulation of Free Droplets,” Nature 426:515-516 (Dec. 2003) discloses a dielectrophoretic transporter for moving droplets suspended in oil by timed switching of a series of electrodes.
Another type of nanoscale oscillator is described in Sazonova et al. “A Tunable Carbon Nanotube Electromechanical Oscillator,” Nature 431:284-287 (16 Sep. 2004). This publication describes a nanotube grown over a trench between two metal (Au/Cr) electrodes. Nanotube motion is caused by interaction with a gate electrode at the bottom of the trench.
A device that fully realizes surface tension's great advantages should be nanoscale, where surface tension is strongest relative to other forces, and involve a controllable, reciprocating mechanical action. Described below is the utilization of surface tension forces in a nanoelectromechanical relaxation oscillator. Relaxation oscillators, which relate to such diverse phenomena as heartbeats (Ref 13), leaky faucets (Ref 14) and earthquakes (Ref 15), are generically characterized by two distinct time scales: a fast relaxation phase and a slow recovery phase. In our oscillator implementation, the fast relaxation is a hydrodynamic event driven by surface tension. The slow portion of the oscillator cycle is electrically driven and involves atom-by-atom transport, or “mass transport.” (Ref 16). By avoiding hydrodynamic transport during the slow half-cycle, direct confrontation between the electrical and surface tension forces is circumvented. Thus this device accesses the full strength of the surface tension forces during relaxation, yet only requires low voltages for operation.